Measurement of electrical resistivity beneath the seafloor has assumed an important role in hydrocarbon, e.g., oil or natural gas, exploration and reservoir assessment and development. Seismic methods have traditionally been used for such purposes, however, the results can be ambiguous. Several electromagnetic methods have been developed for mapping sub-seafloor resistivity variations. See, for example, U.S. Pat. No. 5,770,945 of Constable (magnetotelluric (MT) methods), WO 03/104844 of Constable (MT and controlled electromagnetic (EM) source methods), WO 04/53528 of Constable (controlled EM source for monitoring), U.S. Pat. No. 6,522,146 OF Srnka (controlled EM source), International Publication No. WO 03/048812 of MacGregor and Sinha (controlled EM source), and International Publication No. WO 01/57555 of Rees (controlled EM source). The disclosure of each of the identified patent documents is incorporated herein by reference.
The magnetotelluric (MT) method is an established technique that uses measurements of naturally occurring electromagnetic fields to determine the electrical resistivity, or conductivity, of subsurface rocks. An MT survey employs time series measurements of orthogonal components of the electric and magnetic fields, which define a surface impedance. This impedance, observed over a broad band of frequencies and over the surface, determines the electrical conductivity distribution beneath that surface, with horizontal layers of the earth being mathematically analogous to segments of a transmission line. Principal factors affecting the resistivity of subsurface materials include temperature, pressure, saturation with fluids, structure, texture, composition and electrochemical parameters. Resistivity information may be used to map major stratigraphic units, determine relative porosity or support a geological interpretation. A significant application of MT surveying is oil exploration. An MT survey may be performed in addition to seismic, gravity and magnetic data surveys. A combination of data from two or more different survey methods leads to a more complete understanding of subsurface structure than may be possible through the use of any single technique alone, particularly where the structure is such that measurement using a given technique may be contraindicated.
For example, certain structures such as sediments buried under salt, basalt or carbonate have poor seismic performance and productivity. These structures generate strong reflections and reverberations, making imaging of the buried sediments difficult using acoustic methods alone. On the other hand, because the MT method does not involve the measurement of responses to artificially-created seismic events, it can be utilized in lieu of or in combination with seismic methods to minimize the error induced by reflections.
Another type of electromagnetic field measurement utilizing controlled EM source (CSEM) methods is well known the art and has become almost routine for mapping of electrical conductivity of the seafloor in very shallow to deep ocean water, achieving seafloor penetration depths as great as 30 km in 5 km of water. Such methods have provided significant economic savings in terms of avoiding the costs of drilling test wells into sub-seafloor structures that do not contain economically recoverable amounts of hydrocarbon.
The current technologies described above typically require antennas of several meters in length (usually about 10 m or longer) to make low noise measurements of seafloor electric fields. Seafloor deployment and recovery of systems with these long antennas can be difficult, and the antennae are subject to damage during handling.
Another disadvantage of existing technologies is that releasable anchors such as the one described in U.S. Pat. No. 5,770,945 are sometimes prohibited as part of the permitting process for seafloor exploration in sensitive areas. The inability to use such anchors introduces significant challenges for deployment and recovery of survey units.
Compact underwater sensors have been developed for applications such as underwater surveillance and submarine detection. One such sensor is the three-axis underwater electric field sensor available from Ultra Electronics Holdings plc of Staffordshire, England. This sensor is effective for detection of electronics fields resulting from submarine or ship movement, where accurate quantitative measurement is not required. Because the sensor construction causes distortion of the electric fields and it is not easily calibrated, this sensor is not capable of providing the precise resistivity measurements needed to hydrocarbon reservoir surveying.
Accordingly, the need remains for a survey system capable of measuring seafloor electric fields without requiring the use of long antenna extending along one or all axes of measurement and which is more easily deployed and recovered. The present invention provides a significant advantage in allowing seafloor electric fields to be measured using a much smaller sensor (one meter or less) that can be easily deployed and retrieved using a remotely operated vehicle (ROV), which is logistically very attractive.